A fume hood is a ventilated structure that provides a controlled space in which a lab worker (where “lab worker” refers to a lab worker, lab technician, or any person working in a laboratory whether in a chemical company laboratory, a university, hospital, or other institution) performs experiments with potentially toxic chemicals. The controlled space is partially enclosed in the fume hood structure, which limits exposure to chemicals, as well as other noxious fumes, vapors, or dust as the lab worker works while positioned outside of the fume hood. The lab worker is provided access to the controlled space through a sash opening. The sash opening can be adjusted by moving sash panels (also referred to as fume hood doors, sash doors, sashes) that adjustably cover an opening in the fume hood. An exhaust fan draws air from the room through the sash opening into the work area in the controlled space. The air is then vented outside of the building by the exhaust fan thereby removing the fumes, vapors or dust. The amount of air required to contain the chemicals is related to the open area of the sashes that are between the user and the chemicals. The greater the open area, the more air flow that is required to contain the chemicals.
The area of the sash opening in fume hoods may be controlled by the user. Fume Hood Controllers are being provided to measure the position of each sash panel, and to use the sash positions to determine the total open area of the fume hood. The Fume Hood Controller then uses the total open area, the measured exhaust flow, and the user-defined face velocity set point to maintain the required volume of airflow through the fume hood. The required volume of airflow is the volume of airflow that is sufficient to maintain the chemicals in the fume hood.
Fume hood controllers typically include a suitable processor and supporting memory, and permits entry and storage of the dimensions of the sash panels and other structural features. A sensor or multiple sensors are provided at strategic locations in the fume hood to determine the position of each sash. The open area of the fume hood is determined using the position of the sashes and the dimensions of the structure according to the geometry of the sash opening and fume hood.
The sensors used to detect the positions of the sashes typically require attachment or mounting by rather complex structure that is typically difficult to install. One example sensor structure uses conductive strips layered opposite a resistive strip and adhesively mounted on an edge of a sash. An actuator block is movably mounted in a track on a base member mounted to the fume hood and extending along a length in parallel with, and in suitably close proximity to, the edge of the sash. The actuator block is linked to a mounting block affixed to the sash such that when the sash panel moves, the linkage moves the actuator block in the track of the base member. As the actuator block moves within the base member, a steel ball that is spring loaded in the actuator block presses the conductive strips together. The conductive strips are energized in a voltage divider circuit and when the steel ball presses on the conductive strips, a short circuit is created and the position of the steel ball may be determined by taking voltage measurements.
Another way of sensing the position of fume hood sashes involves positioning a string of light emitters and corresponding detectors on a surface of the fume hood in proximity to one side of a sash. The light emitter and detector pairs are mounted along the length of one side of the sash and the detector is connected in parallel with a resistor. The string of light emitters and corresponding detectors is energized and the signal level at the end of the series connection of emitters is monitored. When the sash is present at a given light emitter, the light from the emitter is reflected off the side of the sash and the reflected light is detected by its corresponding light detector. When the detector senses the light reflected off the sash surface, the resistor in parallel with it is bypassed causing a corresponding change in the signal level across the series connection of the detectors. The signal level changes as the sash moves and provides an indication of the position of the sash.
One problem with this method is that the reflected light may not be reliably sensed if the sash surface is not sufficiently reflective. Even if special reflective surfaces are provided, the surfaces may be subject to deterioration from exposure to fumes, which may reduce the reflectivity of the surface. In addition, installation of the system may be difficult where sash panel frames may vary substantially with regards to the distance between frame surfaces and sash surfaces. In fume hoods having several layers of sashes opening and closing, the light reflection may be suitable for the near sashes, but diminish for sashes that are further away. This may result in a failure to detect a closed sash.
Another way of sensing the position of the fume hood sashes involves mounting a string potentiometer to the fume hood. An end of the string potentiometer is fixed to the sash so that the sash pulls the string and changes the resistance on the potentiometer. A voltage measurement determines the extent to which the sash has pulled the string and when properly calibrated, provides the position of the sash.
The apparatuses and methods currently used for determining the position of the sash on a fume hood suffer from requiring mounting structure for sensing mechanisms that is difficult to install. The currently used apparatuses and methods typically require custom installation and designing the solution into the fume hood. The apparatuses and methods are also difficult to calibrate. The difficulty in installing and calibrating the sensing mechanisms affect the reliability, cost and robustness of the position sensing apparatuses.
In view of the foregoing, there is an ongoing need for systems, apparatuses and methods for determining the position of sashes on fume hoods that involve structure that is easy to install and calibrate.